The invention releates to a method and apparatus for measuring the misregistration between a record/playback transducer and an information track on a recording medium. For example, the invention may be used to measure head-track misregistration in a direct access storage device such as a magnetic disk file.
In a magnetic disk file, information is stored on a magnetizable disk along concentric circular tracks. The amount of information whicch can be stored on a disk depends, in part, on the number of tracks on the disk. Therefore, one way to store more data on a disk is to increase the track density.
As adjacent tracks on the magnetizable disk are moved closer together, the record/playback transducer (the record/playback head) must be positioned more precisely over the track of interest in order to avoid cross talk between adjacent tracks. Therefore, it is important to minimize the amount by which the record/playback transducer wanders off of the center of the track. This amount of wandreing is known as track misregistration.
There are many factors which are responsible for head and track misregistration. These factors may be divided into two categories: static and dynamic. Static misregistration includes effects which vary slowly in time, such as thermal drift and disk creep. The static components of head and track misregistration may be removed with modest servo requirements.
Dynamic head and track misregistration includes effects which cause the head to change its position relatively quickly such as in less time than it takes to read or write a single track. Causes of dynamic misregistration include nonrepeatable runout of the disk rotation bearings, external vibrations, mechanical interactions in the disk file, and transient vibrations such as hed settle-out after the head is moved to a new track.
The removable of dynamic components of head and track misregistration by servoing requires wide bandwidth servos which are expensive and difficult to construct. Therefore, disk file designs which minimize dynamic track misregistration components are highly desireable.
While individual components of head and track misregistration may be small, the cumulative effect of all causes of misregistration may be troublesome. Hence, it is desireable to minimize each component of head and track misregistration.